Hello Gene,
To start with the end of your reply, I actually am a mathematician :)
On 7/9/20 4:24 PM, Gene Heskett wrote:
On Thursday 09 July 2020 14:23:57 R C wrote:
Hello,
this is (probably) off topic, been seen that happen. If it is please
ignore it.
I am building a "motorized" telescope mount (dobsonian) with what is
called an equatorial platform, it has 3 axis which I am going to drive
with stepper motors.
The stepper motors I use with a stepper driver, those common DM542
ones, the stepper motors themselves are 2A and 1.8 degrees per step.
What I want to accomplish with the equatorial platform) (it
compensates for the rotation of the earth) is that, the start and end
position accuracy is not that important, smooth and
constant/consistent movement is. for the azimuth/altitude precision
is not a really big deal, but you'd want to move these 2 axis
somewhat swift.
So there are a few factors to decide.
I probably want micro stepping, what settings on the driver for
pulses per rev, is best to use (or is that just trial and error?)
I think the gear ratio is as important as the microstepping ratio which I
would probably set to 8 or 16 with the dip switches on the dm542's. Any
higher than that and the higher speeds need stepper pulses faster than
the opto's in the DM542 inputs which will limit your top speed to just
north of 200 kilohertz for a step rate.
Correct, the gear ratio and stepper resolution are important, so is
leverage/torque.. and well.. space to put it. To move the mount
on it's vertical axis, azimuth, and to move the newtonian tube, the
latitude, U use 360 teeth gears, because they are bigger and have more
"leverage", so
I can move them easier/faster (somewhat, that is a trade off too).
Next is the resolution of your mirror, so you would want it to move about
4 microsteps to see it visibly move in the eyepiece.
The one I am doing this with is not that big, it is an 8" main mirror.
Then if you are well balanced and running on ball bearings for low
friction on all 3 axis's, you may want to experiment with the motor
current to see if a certain current gets you a more even movement, some
drivers, and I haven't asked, nor tried to prove that 1/8th or 1/16 step
gives the same movement as it depends both on the currant mapping used
by the DM542 driver and the magnetic (iron) properties of the motor.
Yes they all have ball bearings, a big "lazy susan" style one betwen
platform and mount, pillow/pocket bearings for the
elliptical friction "runners". However, kepping the Azimuth
bearing/joint and latitude bearing/joint steady for a dobsonian is more
important.. they kinda need to be "locked" when on a target.
The equatorial platform needs to run smoothly and steady, no or little
"jerking"
That is something I want to experiment/try/figure-out if there is a
micro stepping setting that would be best to use. The
smoother/constant/consistent I mover the EQ
platform the better. That is where micro stepping and speed comes in I
think. If I can continuously can keep moving without stopping, slowing
down, speed up, the better it is probably.
As with PWM itself, I am probably just not too familiar with it. From
what I understand, the voltage I use for the motors determines how
fast I can go (I am going to use a 48V switching power supply).
Steppers aren't pwm driven, so ignore that. But a 48 volt supply is
pushing the ratings of a DM542 which is 50 volts max, and you would be
wise to adjust it a few volts lower. I have a pair of 7.5 amp 48 volt
supplies running my 11x56 Sheldon lathe, but they are turned down to
42.5 volts, much safer, and it still marches at way faster speeds than
needed for a good cut.
I can do that, I can put a voltage regulator between them. What do you
mean by "safer" btw?
as for PWM, I can of course change the length of the pulse itself
and, independently, change the time between two pulses. What is the
relation ship there? WHat does a longer width of the pulse itself
do? and what exactly does a longer gap between the pulses do (of
course the wider the gap between two pulses the slower the motor
turns).
for, especially, the equatorial platform, I want to avoid "jerking"
it, meaning starting and stopping the stepper motor as little as
possible and just go at a 'slow' constant speed.
LinuxCNC has "adjustment knobs" for that, but are set with a text editor
and forgotten once done. Slewing to another star can be done with a
second or more to get to speed, and a slowdown in approaching the new
target so "jerk" can be very low.
I am not too worried about that part, zooming in on an object, that is
fine, I could do that manually, well remotely. and once "locked in"
keep it steady and start the EQ platform.
I am also terribly worried about the "Go To" features of the telescope.
I am planning on a good camera on the telescope, and a "cheap" camera
on the finder scope. Ultimately I want to use Stellarium, and maybe
even use the Stellarium connection features to connect to the telescope
(this project is going to take a while).
The problem I see in moving a Dob is in the kinematics conversion because
its axis's for az-el need to be mapped to the location on the planet for
the base motors movements. A newtonian on a polar mount cancels 99.99%
of that because the 2 axis's are 90 degrees to each other, the Dob mount
is not.
That is correct, but that would be a "way later" thing to play with..
that part would definitely need some geometry and math trickery.
All the "pivoting" points are offset to eich other... when the
platform moves, the whole telescope is tilting, etc etc etc...
sorry if totally of topic....
Not totally. Might be an interesting diversion. :) But I'll let some of
our better maths people comment on the kinematics.
Cool, well I am glad you see it like this. Basically what I am thinking
is, OK, my mill has 3 stepper drivers/motors, and that telescope does
too (once I built it like that)... and .. same hardware, same motors,
same stepper-drivers... So "these emc-user" guys must know how to
work that hardware.. because both my Sherline mill and lathe are
working just fine using it with the same hardware :)
Ron
thanks,
Ron
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Cheers, Gene Heskett
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